Emission occurring only during irradiation of a substance with light, that is, only under stimulation with light, is generally called fluorescence and a substance with this property is called a fluorescent substance. Emission lasting even after termination of stimulation of the substance with light is called phosphorescence or afterglow, and a substance with this property is called a phosphorescent substance. There is a phosphorescent substance observed to exhibit afterglow for dozens of minutes to several hours after termination of light stimulation, and a substance with this property is called a photostorage or long afterglow florescent substance. Accordingly, a pigment containing the fluorescent substance emits light only during irradiation with light, while a pigment containing the photostorage fluorescent substance, unlike the fluorescent pigment, emits light over a long time even after termination of irradiation with light. Such a pigment is called a photostorage pigment.
Conventionally, an ink composition using such photostorage pigment is known. For example, JP-A-9-53034 (1997) and JP-A-9-59555 (1997) describe respectively a photostorage ink composition using a long afterglow florescent substance (Sr.Al2O4: Eu) comprised of a compound represented by the formula Sr.Al2O4 as a mother crystal and europium (Eu) as an activator. JP-A-2000-104001 and JP-A-2000-109740 describe respectively a photostorage ink composition using a long afterglow fluorescent substance (Sr.Al2-xBxO4: Eu) comprised of a compound represented by the general formula Sr.Al2-xBxO4 wherein x is a numeral in the range 0.1≦x≦1 and europium (Eu) as an activator.
However, such conventionally known photostorage pigment is chemically unstable and is particularly lacking water resistance so that when it is used as a coloring agent in an ink composition, a nonpolar hydrocarbon solvent such as xylene should be inevitably used, and there is a problem that upon subjection to writing on usual paper, such ink composition causes bleeding. Because the nonpolar hydrocarbon solvent is used as a dispersing medium of the photostorage pigment as described above, there is another problem that a container or a holder for encapsulating such ink composition is limited to the one consisting of a solvent-resistant material. Further, the conventional photostorage pigment has practically various problems; for example, there is a problem that after termination of irradiation with light, the duration of afterglow is so short that the photostorage pigment cannot be used in applications requiring long-lasting emission in the nighttime.
Conventionally, a writing instrument with the above-described photostorage ink composition charged into a container is also known. This writing instrument contains the above-described ink composition charged and sealed in an opaque container made of resin, so that while the writing instrument is not used, the ink composition in the container is not irradiated with light, and when the writing instrument is used in writing, the resulting handwriting is irradiated for the first time, and therefore there is a problem that the handwriting does not emit light not only during writing but also just after writing.